In recent years, due to the growing interest in energy saving, there is a high demand for an infrared shielding film which shields transmission of heat rays of sunlight by being installed in the windowpane of the buildings or the vehicles from a viewpoint of reducing the load applied to air conditioning facilities.
Light emitted from the sun has a broad spectrum covering from the ultraviolet region to the infrared region. Visible light has the wavelength range of 380 to 780 nm from violet, yellow, to red light, and it accounts for about 45% of the sunlight. With regard to infrared rays, those close to the visible light are referred to as near infrared rays (wavelength of 780 to 2500 nm) and those above the range are referred to as intermediate infrared rays, and they account for about 50% of the sunlight. Although the intensity of the light energy in the range is about less than 1/10 of ultraviolet rays, it has a strong thermal action, and thus when absorbed by a material, it is released as heat to cause temperature increase. From this point of view, it is also referred to as heat rays. By shielding those rays, room temperature increase can be suppressed. It is also possible to suppress dissipation of heating energy to outside the room during the winter in a cold area.
Mainly, a dry film forming method such as a vapor deposition method or a sputtering method, has been proposed as the method of producing an infrared shielding film in which laminate films are formed by laminating the high refractive index layer and a low refractive index layer by turns. However, since the dry film forming method needs a large size vacuum equipment for the forming, there are problems, for example, that it requires high manufacture cost so that it is difficult to form a film with a large area, and the substrate is limited to a heat resistant material.
Instead of the dry film forming method having the problems like above, a method of forming an infrared shielding film by a wet coating method is known.
For example, a method, of forming a transparent laminate using a coating liquid for a high refractive index layer in onion a thermocurable silicone resin or a UV curable acryl resin containing microparticles of a metal oxide or a metal compound is dispersed in an organic solvent and coating it on a substrate according to a wet coating method using a bar coater is known (for example, see Patent Literature 1).
Further, a method of forming a transparent laminate by coating a composition for forming a high refractive index coating film which consists of rutile type titanium dioxide, a heterocycle-based nitrogen compound, (e.g., pyridine), a UV curable binder, and an organic solvent on a substrate according to a wet coating method using a bar coater is disclosed (for example, see Patent Literature 2).
Meanwhile, as a method of forming a layer containing titanium dioxide particles, for example, a method of laminating by turns using a slurry containing particles of spherical rutile type titanium dioxide dispersed in methanol and methanol silica sol is known (for example, see Patent Literature 3).